1. Field of the Invention
The present invention relates to a process for making a work piece from a β-type titanium alloy material that is capable of building up the hardness distribution with the hardness continuously varied across the surface of the work piece, as intended by a designer.
2. Discussion of the Background
In recent years, a demand for lighter weight and higher stiff parts is increasing in light of energy savings and environment protections. In order to meet this demand, a demand for developing a high performance material with a hardness gradient in a certain region of the material is increasing for the purpose of designing the hardness distribution suitable for each part or member.
Particularly for a plate, if the hardness and the plate thickness can be simultaneously applied as intended by a designer, bending strength, ability of vibration restraint, performance of bend or the like can be freely imparted as designed. It is needless to say that a plate with various characteristics thus imparted can be widely used.
As a conventionally used technique to impart different characteristics to the inside of a member or part, there is a technique to apply a so-called clad material formed by combining different metals (prior art 1). For example, in order to produce a clad steel plate, two or three different metal plates are layered and hot rolled into a single plate imparted with complex characteristics. This technique has come into practice for the purpose of imparting complex characteristics including corrosion resistance characteristic, magnetic characteristic, light weight characteristic and the like, rather than building up a specific hardness distribution in a member or part.
On the other hand, varying the hardness depending on the position in the same material is achievable by a hardening treatment for increased hardness, a annealing such as a spherodite annealing for decreased hardness, or other heat treatment, in which various heat conditions are employed for each intended purpose. Particularly, induction hardening treatment allows hardening of only the surface of a material, thereby achieving improved surface wear resistance (prior art 2).
It is known that a β-type titanium alloy material is subjected to cold working and aging treatment so as to have hardness controlled for increased hardness. Japanese Patent Application Laid-open No. 2001-54595 (prior art 3) discloses that a cold working ratio is set to 15% or higher and cold working is carried out in combination with the aging treatment. According to the teaching of this prior art 3, there is provided a desirable effect in increasing the hardness of a metal plate for assuring a required material hardness and hence the durability against crack.
As described above, the prior art 1 is directed to a technique to hot rolling of two or three different metal plates layered together into a single clad steel plate, to which complex characteristics are imparted. This means that the prior reference 1 only teaches layering of different plates, which makes it possible only to arrange materials respectively having two or three hardness conditions across the surface of a plate. Accordingly, the hardness distribution with the hardness continuously or gradually changed across the surface of a plate and hence a desired hardness distribution across the surface thereof is highly unlikely to be built up by this technique. Also, there are other problems such as difficulty in assuring hardness across the bonded interface, increased manufacturing costs and the like. Therefore, this technique is applicable only to a limited area.
In building up the hardness distribution across the surface of a material by the induction hardening treatment of the prior reference 2, although regions with different hardness values are obtainable by having regions contacting and not contacting a induction coil, it is very difficult to carry out the induction hardening treatment by precisely controlling the positioning of the induction coil corresponding to each intended region and the value of the hardness to be applied to each region. Accordingly, this induction hardening treatment is not suitable for building up the hardness distribution across the surface of a material.
Although the prior reference 3 discloses subjecting the cold working and the aging treatment to a β-type titanium alloy material for improved hardness in the entire metal plate aiming at improvement in durability against crack, it neither teaches nor suggests a technique to build up the hardness distribution across the surface of a material.
It is an object of the present invention to provide a process for making a work piece from a β-type titanium alloy material with the hardness continuously or gradually changed across the surface.